Such applications as document scanners, free-space optical interconnections, and optical displays require simple, compact and efficient illumination systems that can illuminate an object with a desired irradiance profile using a narrowband and spatially-incoherent light source. Such light sources are lower in cost than coherent light sources, such as lasers. Energy efficiency is important in all applications, since the cost of the power supply that powers the light source can be a major factor in the cost of making such devices. In portable devices, energy efficiency is of paramount importance because of battery size and battery life considerations.
Some types of known manually-operated document scanners illuminate the document using one or more LEDs. LEDs generate a non-uniform irradiance profile, but the document needs to be illuminated with a uniform irradiance profile. A uniform irradiance profile is derived from the non-uniform irradiance profile of the LEDs by interposing a suitably-shaped stop between the LEDs and the document. With this arrangement, the document is illuminated by the small segment of the light output of the LEDs in which the profile is acceptably uniform. The uniform irradiance profile is therefore obtained at the expense of low efficiency, since most of the light generated by the LEDs is absorbed by the stop, and does not illuminate the document.
Techniques for shaping the irradiance profile of a coherent light source using refractive and diffractive optics are known in the art. For example, a way of using two aspheric lenses to shape a coherent and collimated Gaussian beam into a rectangular and uniform irradiance profile is described in U.S. Pat. No. 3,476,463. Other examples of using refractive systems to reshape Gaussian beams are described by P. W. Rhodes and D. L. Shealy in Refractive Optical Systems for Irradiance Redistribution of Collimated Radiation: Their Design and Analysis, 19 APPL. OPT., 3545-3553 (1980) and by C. Wing and D. L. Shealy in Design of Gradient-Index Lens Systems for Laser Beam Reshaping, 32 APPL. OPT., 4763-4769 (1993). Examples of diffractive systems are described by M. T. Eismann, A. M. Tai, and J N. Cederquist in Iterative Design of a Holographic Beam-Former, 28 APPL. OPT., 2641-2650 (1989) and by N. C. Roberts in Multilevel Computer-Generated Holograms with Separable Phase Functions for Beam Shaping, 31 APPL. OPT., 3198-3199 (1992).
In Acousto-optic Conversion of Laser Beams into Flat-top Beams, 40 J. MOD. OPTIC., 625-635 (1993), E. Tervonen, A. T. Friberg, and J. Tarunen proposed using crossed acousto-optic cells to convert a single coherent laser beam into a partially coherent field consisting of multiple, non-interfering beams as a way of generating a desired irradiance profile from a coherent light source. The acousto-optic cells were driven by computer-generated waveforms equivalent to one-dimensional array generators, and so can be regarded as synthetic acousto-optic holograms. The approach described by Tervonen et al. produces a desired irradiance profile, but is complex, bulky, expensive, and consumes additional power to drive the acousto-optical cells.
Accordingly, it would be desirable to have an illumination system that can generate a desired irradiance profile from an incoherent light source, and that is simple, compact and energy efficient.